1. Field of Invention
The present invention relates to a method of manufacturing a nonlinear element, a method of manufacturing an electrooptic device, an electrooptic device, and an electronic apparatus. More specifically, the present invention relates to a technique for forming an insulating film which constitutes a nonlinear element.
2. Description of Related Art
Electrooptic devices, such as a liquid crystal device and an EL (electroluminescence) display panel, have been recently popularized as display sections of electronic apparatuses, such as a cell phone, a portable computer, a video camera, etc.
A known example of liquid crystal devices among such electrooptic devices uses a thin film diode (TFD) element as a pixel switching nonlinear element for active matrix driving.
Such a liquid crystal device can include a pair of substrates which hold a liquid crystal therebetween, TFD elements and pixel electrodes formed on one of the substrates, and stripe-shaped counter electrodes (signal electrodes) formed on the other substrate. Each of the TFD elements is formed by, for example, forming a first metal film such as a Ta film or a Ta alloy film on the surface of the substrate, forming an insulating film on the surface of the first metal film by anodization, and then forming a second metal film on the surface of the insulating film.
In a liquid crystal device using the TFD elements as nonlinear elements, when the current-voltage characteristics of the TFD elements have high nonlinearity, the driving voltage of the liquid crystal device can be set to a low value, and a high-contrast display can be realized. Based on this view, a method of annealing an insulating film in a water vapor atmosphere to modify the insulating film has been studied as a technique for improving the characteristics of the TFD elements, as disclosed in, for example, Japanese Unexamined Patent Publication No. 08-830953. However, further improvement is required.
In order to suppress the difference (polar difference) in quantity between the currents flowing through a TFD element with the positive and negative applied voltages, a back-to-back structure is generally used, in which two TFD elements are connected in series. In the back-to-back structure, the step of depositing an insulating film by anodization requires power feed to an element portion, and thus anodization must be effected in a state in which a portion constituting a TFD element is connected to wiring for passing an anodization current through a bridge portion. Therefore, after the insulating film is formed by anodization, the bridge portion for connecting the TFD element portion and the feed wiring for anodization must be cut, thereby causing the problem of increasing the number of manufacturing steps.
In consideration of the above problem, an object of the present invention is to provide a method of manufacturing a nonlinear element capable of further improving electrical properties, such as nonlinearity of a TFD element, a method of manufacturing an electrooptic device, an electrooptic device, and an electronic apparatus.
Another object of the present invention is to provide a method of manufacturing a nonlinear element capable of decreasing the number of steps for manufacturing a nonlinear element to permit the manufacture of a nonlinear element at low cost.
In order to achieve the objects of the present invention, a method of manufacturing a nonlinear element having a first metal film, an insulating film, and a second metal film, which are laminated in this order, can include the first metal film forming step of forming the first metal film, the second metal film forming step of forming the second metal film, and the high-pressure annealing step of performing annealing in a predetermined atmosphere under a high pressure between the first metal film forming step and the second metal film forming step.
In the present invention, in order to modify the insulating film formed by anodization, high-pressure annealing is performed in an inert gas atmosphere, a nitrogen gas atmosphere or an atmosphere containing water vapor. In order to oxidize the surface of a metal film, high-pressure annealing is performed in an atmosphere containing water vapor. In this way, high-pressure annealing in a predetermined atmosphere can further improve the nonlinearity of the current-voltage characteristics of a nonlinear element, as compared with normal-pressure annealing in a water vapor atmosphere. Therefore, in a liquid crystal device, the driving voltage can be set to a low value, and a display of high quality such as improved contrast or the like can be realized.
In the present invention, the first metal film can include, for example, a metal film containing at least Ta.
For example, the present invention may include the first metal film forming step, an insulating film forming step of forming the insulating film on the surface of the first metal film by anodization, the high-pressure annealing step of performing annealing in an inert gas atmosphere, a nitrogen gas atmosphere or an atmosphere containing water vapor, and the second metal film forming step for forming the second metal film on the surface of the insulating film. In this case, high-pressure annealing is performed, for example, under the conditions including a temperature of 100xc2x0 C. to 600xc2x0 C., and a pressure of 0.5 MPa to 3 MPa, and preferably the conditions including a temperature of 150xc2x0 C. to 300xc2x0 C., and a pressure of 0.5 MPa to 2 MPa. In this construction, the insulating film formed by anodization can be modified to improve the nonlinearity of the current-voltage characteristics of a nonlinear element. Also, high-pressure annealing can obtain the same effect as conventional annealing at a lower annealing temperature than the conventional annealing, thereby preventing deformation of a substrate.
For example, in experiment for manufacturing a TFD element having the back-to-back structure, an anodic oxide film was formed on a Ta film by anodization with a voltage set to each of 10 V, 15 V and 20 V, and then high-pressure annealing for 1 hour at 200xc2x0 C. in an atmosphere containing water vapor under a high pressure of 1 MPa, high-pressure annealing for 1 hour at 300xc2x0 C. in an atmosphere containing water vapor under a high pressure of 1 MPa, and high-pressure annealing for 1 hour at 350xc2x0 C. in a nitrogen atmosphere under a high pressure of 1 MPa were performed for studying nonlinearity xcex2 of a nonlinear element. As a result, the results shown by solid line C, point B and solid line D in FIG. 1(a) were obtained. As shown in FIG. 1 (a), good nonlinearity xcex2 can be obtained by high-pressure annealing. According to the experimental results shown in the graph, and other experimental results obtained, high-pressure annealing at a temperature of 200xc2x0 C. and a pressure of about 1 MPa produces a nonlinear element having good nonlinearity xcex2.
For example, the present invention can include the first metal film forming step, the high-pressure annealing step performing high-pressure annealing in an atmosphere containing water vapor to oxidize the surface of the first metal film, the insulating film forming step of forming the insulating film on the surface of the first metal film by anodizing the surface of the first metal film, and the second metal film forming step for forming the second metal film on the surface of the insulating film. In this case, high-pressure annealing is performed, for example, under the conditions including a temperature of 100xc2x0 C. to 600xc2x0 C. and a pressure of 0.5 MPa to 3 MPa, and preferably the conditions including a temperature of 150xc2x0 C. to 300xc2x0 C. and a pressure of 0.5 MPa to 2 MPa. In this construction, the insulating film formed by high-pressure annealing, and the insulating film formed by anodization can be used as the insulating film of the nonlinear element, thereby improving the nonlinearity of the current-voltage characteristics of the nonlinear element.
For example, the present invention can include the first metal film forming step, the high-pressure annealing step of oxidizing the surface of the first metal film by high-pressure annealing in an atmosphere containing water vapor to form the insulating film, and the second metal film forming step for forming the second metal film on the surface of the insulating film. In this construction, the insulating film formed by high-pressure annealing can be used as the insulating film of the nonlinear element, thereby improving the nonlinearity of the current-voltage characteristics of the nonlinear element. Furthermore, since the insulating film is formed by high-pressure annealing, anodization is not performed. Therefore, the TFD element having the back-to-back structure has no need to form a bridge portion for power feeding in anodization, and thus the step of removing the bridge portion can also be omitted.
For example, as a result of experiment, when anodization of the Ta film is not performed, i.e., when high-pressure annealing at 350xc2x0 C. with an anodization voltage of 0 V is performed, a nonlinear element exhibiting a good nonlinearity xcex2 of about 5.5 is obtained, as shown by point A in FIG. 1 (a). According to the results of experiment, a nonlinear element having good nonlinearity xcex2 is obtained by high-pressure annealing at a temperature of 350xc2x0 C. and a pressure of 1 MPa.
FIG. 1(b) shows the relationship between the high-pressure annealing condition and the thickness of a Ta oxide film formed by oxidizing the surface of a Ta film in high-pressure annealing. Under the condition shown in FIG. 1(b), the atmosphere contained 100% of water vapor. In treatment at a temperature of 350xc2x0 C. and a pressure of 1 Ma for 1 hour, the Ta oxide film having a thickness of about 250 xc3x85 was deposited. While in treatment at a temperature of 350xc2x0 C. and a pressure of 1.5 Ma for 1 hour, the Ta oxide film having a thickness of about 320 xc3x85 was deposited.
In the present invention, after the high-pressure annealing, re-annealing is preferably performed for annealing the insulating film under a lower pressure than that in the high-pressure annealing.
For example, the present invention preferably includes the first metal film forming step, the third metal film forming step of forming a third metal film on the surface of the first metal film, the step of oxidizing the surface of the third metal film by high-pressure annealing in an atmosphere containing water vapor to form the insulating film, and the second metal film forming step for forming the second metal film on the surface of the insulating film. In this case, the third metal film includes, for example, a metal film containing at least Ta. In this construction, the insulating film formed by high-pressure annealing can be used as the insulating film of the nonlinear element, thereby improving the nonlinearity of the current-voltage characteristics of the nonlinear element. Furthermore, since the insulating film is formed by high-pressure annealing, anodization is not performed. Therefore, the TFD element having the back-to-back structure has no need to form the bridge portion for power feeding in anodization, and thus the step of removing the bridge portion can also be omitted.
In the present invention, after the high-pressure annealing, re-annealing is preferably performed for annealing the insulating film under a lower pressure than that in the high-pressure annealing.
In the present invention, when the insulating film formed by high-pressure annealing is used as the insulating film of the nonlinear element, high-pressure annealing is performed under the conditions including a temperature of 250xc2x0 C. to 400xc2x0 C. and a pressure of 0.5 MPa to 3 MPa, and preferably the conditions including a temperature of 300xc2x0 C. to 400xc2x0 C. and a pressure of 0.5 MPa to 2 MPa. By annealing under such a high pressure, oxidization of the surface of the metal film is accelerated to obtain a uniform oxide film having good properties as the insulating film, as compared with annealing under a lower pressure (for example, under normal pressure). Therefore, in the TFD element including the insulating film formed by this method, the nonlinearity of the current-voltage characteristics can be further improved.
In the present invention, re-annealing is preferably performed before the second metal film forming step. Namely, it is preferred that after re-annealing is carried out, the second metal film is formed on the surface of the insulating film in the second metal film forming step.
In the present invention, re-annealing is preferably performed in an atmosphere containing water vapor or nitrogen to crystallize the insulating film. In this construction, the insulating film can be crystallized to improve the nonlinearity of the current-voltage characteristics of the nonlinear element.
In the present invention, re-annealing is preferably performed at a temperature of 100xc2x0 C. to 500xc2x0 C.
In accordance with the present invention, a method of manufacturing an electrooptic device comprising a substrate maintaining an electrooptic material can include forming a nonlinear element as a pixel switching element by the method of manufacturing a nonlinear element of the present invention, and forming a pixel electrode on the substrate so that the pixel electrode is connected to the nonlinear element.
An electrooptic device manufactured by the method of present invention can include a substrate maintaining an electrooptic material, a nonlinear element having a first metal film, an insulating film and a second metal film and provided as a pixel switching element on the surface of the substrate on which the electrooptic material is maintained, the insulating film being formed by annealing the first metal film.
In another aspect of present invention, an electrooptic device manufactured by the method of the present invention includes a substrate maintaining an electrooptic material, a nonlinear element having a first metal film, an insulating film and a second metal film and provided as a pixel switching element on the surface of the substrate on which the electrooptic material is maintained, the insulating film being formed by annealing a third metal film formed on the surface of the first metal film.
In the present invention, the electrooptic material is, for example, a liquid crystal.
In the present invention, the TFD element having good characteristics can be formed by a simple process, and thus the method of manufacturing an electrooptic device using the simple process is capable of manufacturing, by a simple process, an electrooptic device capable of realizing high contrast while keeping the driving voltage down.
An electrooptic device to which the present invention is applied can be used as the display sections of electronic apparatuses, such as a cell phone, a mobile computer, and the like.